Added a new BaseFacebook::disableAppSecretProof() method to stop sending...

... the appsecret_proof parameters with the requests. This should be used only with the 'Require AppSecret Proof for Server API calls' app setting set to 'Disabled'.

Prior to this change, even though the 'Require AppSecret Proof for Server API calls' app setting was set to 'Disabled', the appsecret_proof parameter was still sent by the SDK and checked by the server.